Improve RejectionFlag handling during reintegration

• Peak3D::_integration_flag is a different instance of RejectionFlag in Peak3D
• _integration_flag stores reason for rejection only during integration
• On reintegration, _integration_flag is reset, since integration bounds have a decisive effect on whether a peak is rejected
• Peak3D::_rejection_flag remains and is in principle immutable once set
• Number of rejected flags will no longer decrease the more times a PeakCollection is reintegrated (manifested as decreasing completeness)
• Rejection of overlapping peaks and masked peaks is now done during integration, and is reflected via Peak3D::_integration_flag, and can thus be reset
• Added titles to most anonymous dialogue boxes.

Resolves #585 (closed), #586 (closed)